1. Field of the Invention
The present invention generally relates to a novel method for the preparation of maleimides. More specifically, the present invention relates to a method involving direct imidation of maleic anhydride and a primary amine to afford maleimides.
2. Description of the Related Art
Maleimides and their derivatives are widely used in various industrial fields, such as synthetic resins, medicine, agricultural chemistry, and heat-resistant polymer modifiers or photo-electrical materials.
A typical method for preparing maleimides uses direct amination of maleic anhydride and a primary amine to produce maleamic acid intermediate, followed by dehydrated cyclization (imidation) with a dehydrating agent (e.g., acetic anhydride). By this method, however, each mole of maleimide needs one mole of acetic anhydride and produces two moles of acetic acid, which causes corrosion in the equipment and result in complex follow-up treatments and environmental pollution. Such method has been disclosed in U.S. Pat. No. 2,444,536, in which acetic anhydride and sodium acetate are used as catalysts.
Another method disclosed in UK Patent No. 1,041,027 and U.S. Pat. No. 3,431,276 utilizes an acidic catalyst and an organic solvent with a boiling point higher than 80xc2x0 C. in an azeotropic distillation system for preparing maleimides, wherein water produced is constantly removed. The organic solvent can be toluene, xylene, or chlorobenzene, and the catalyst can be sulfur trioxide, sulfuric acid or phosphoric acid. Although this method requires a lesser amount of the expensive dehydrating agent, the production yield is not satisfactory, thus rendering the process uneconomical.
Furthermore, it is disclosed in Japanese Patent Publication No. 53-68700 and Japanese Patent Publication No. 57-42043 that adding an aprotic polar solvent can improve the solubility of the maleamic acid intermediate, and consequently results in a higher yield. However, aprotic polar solvent is highly poisonous and difficult to remove because of its high boiling point.
Japanese Patent No. 54-30155 discloses a high yield method using quaternary amine salts of organic or inorganic acids as catalysts to synthesize oligmers of maleimide. In this method, a certain ratio of quaternary amine salts/acids must be maintained to exhibit satisfactory activity, but such ratio varies in recycled catalysts, and therefore the recycled catalysts must be purified and modified to attain the required ratio. This adds further complexity to the process and results in higher production costs.
In addition, Japanese Patent Publication No. 60-109562 discloses a method using a solvent containing a certain ratio of non-polar solvent and polar solvent, and amino salts of p-benzosulfonic or o-benzosulfonic acid as catalyst, wherein the polar solvent improves the solubility of maleamic acid and catalyst to boost the production yield. However, the polar solvent makes the product, catalyst and solvent in the same phase, thereby increasing the cost of purifying the product.
Furthermore, a small amount of gel byproduct (oligmers) will be produced during the maleimide reaction, which is soluble in reaction solution, but precipitates out when the solution is cooled to below 80xc2x0 C. In this case, the gel byproduct remains in the catalyst, and affects the activity of the catalyst. Although Japan Publication No. 5-213869 discloses that water can be used to recycle carriers; a new catalyst must be added to compromise those lost during recycling. This again further adds complexity to the process, making the process uneconomical.
Therefore, an object of the present invention is to provide a method for producing maleimides using solid acidic catalyst instead of liquid acid catalyst or liquid acid catalyst mixed with solid carriers. According to the invention, maleic anhydride and a primary amine are reacted in the presence of organic solvent to produce maleimide on a solid acidic catalyst. The reaction temperature is preferably between about 100 and 180xc2x0 C., and the molar ratio of maleic anhydride/primary amine is preferably 0.6xcx9c1.8. After the reaction is completed, the catalyst is separated and removed from the solvent to obtain crude maleimide. Then, a high purity maleimide can be obtained by a two-step extraction and crystallization.
The above and other objects are achieved by reacting maleic anhydride and a primary amine at about 100-180xc2x0 C. in an organic solvent and in the presence of a solid acidic catalyst, wherein the molar ratio of the primary amine to the maleic anhydride is about 0.8-1.6.
The primary amine can be straight chain, branched, cyclic, or aromatic amines containing 1-8 carbon atoms. Preferred organic solvents include but are not limited to toluene and xylene.
The method of the invention may further comprise: (I) extracting and crystallizing with an organic solvent having a polarity of less than 0.05; and (II) extracting and crystallizing with an organic solvent having a polarity of greater than 0.2. The organic solvent having a polarity of less than 0.05 is n-hexane or n-pentane, and the organic solvent having a polarity of greater than 0.2 is toluene or xylene.
The solid acidic catalyst is preferably an inorganic acid supported in a neutral carrier such as silicon oxide, zirconium oxide, diatomite or silica gel, and the Hammett Index of the neutral carrier should be greater than xe2x88x922. The inorganic acid can be sulfuric acid or phosphoric acid. The solid acidic catalyst is preferably prepared by roasting at about 200xcx9c400xc2x0 C. The preferable amount of the catalyst is about 50xcx9c200 wt % relative to maleamic acid.
The maleimide reaction is carried out in a low-polarity organic solvent having a boiling point between about 100 and 250xc2x0 C. Upon the completion of the reaction, the solid acidic catalyst and the reaction mixture are filtered at the reaction temperature or below the reaction temperature. The solid acidic catalyst is then set aside for recycling, and the crude maleimide is obtained by evaporation of the solvent. Next, by distilling the crude maleimide under reduced pressure, a high purity maleimide can be afforded.
However, since maleimide is not thermally stable, it deteriorates easily when distilled. To overcome this problem, the present invention features two-step extraction and crystallization to purify crude maleimide. The two steps include: (I) using an organic solvent with a polarity of less than 0.05 as the extractant; and (II) using an organic solvent with a polarity of greater than 0.2 as the extractant. The maleimide thus purified reaches a purity of greater than 99.5 wt %. The organic solvent with a polarity of less than 0.05 is, for example, n-hexane or n-pentane; and the organic solvent with a polarity of greater than 0.2 is, for example, toluene or xylene.
One important advantage of the invention is that this method is applicable in both batch or continuous reactors to attain high yield. In addition, the separation of product and catalyst is much easier. This is because the solid acid catalyst and the solution exist in different phases (solid phase vs. liquid phase) during the reaction. Accordingly, the catalyst and the solution can be separated at various temperatures, and the recycled solid acidic catalyst still exhibits good activity and selectivity. Furthermore, it is viable to separate the catalyst from the solution after cooling the reaction mixture to room temperature. In other words, either at room temperature or reaction temperature, the catalyst can be easily and fully separated for recycle.
In addition, according to the method of the present invention, the gel byproducts produced when cooling the reaction solution to room temperature will segregate out and be separated from the reaction solution automatically. Therefore, there is no byproduct deposit on the catalyst surface, thus avoiding the above-mentioned problem of catalysts losing activity when recycled.
Reference is now made in detail to embodiments of the present invention that illustrate the best mode presently contemplated by the inventor(s) for practicing the present invention.